HYDROGEN. 37 



acid ; and also as a direct agent of the chemical 

 vital processes in the plant. 



The sources for the supply of oxygen to the 

 plant, are carbonic acid, water, and nitric acid. 

 These compounds offer to plants much more 

 oxygen than they require, in consequence of 

 which the most of the oxygen is again restored 

 to the atmosphere; the plant retaining the carbon, 

 a portion of the hydrogen, and the nitrogen of 

 the nitric acid. 



The amount of oxygen exhaled from plants is 

 very considerable ; it is, and must be equivalent 

 to the quantity required for the purpose of 

 respiration, combustion, and decay. If it were 

 not so, the amount of this vital air in the 

 atmosphere would either be increasing or dimin- 

 ishing, either cause unfitting it for the purpose 

 of respiration. 



If the law which governs the liberation of this 

 substance were but slightly changed, an extinc- 

 tion of all plants and animals would follow. 

 Increase or diminish in any sensible degree the 

 vital air, or oxygen, and all vitality must shortly 

 cease. 



HYDROGEN. 



Hydrogen was discovered by Cavendish, in 

 1776. It is one of the most diffused bodies, and is 

 transparent, odorless, tasteless, and inflammable. 



